Evaluation of estrogen and progesterone receptor (ER, PgR) status in breast cancer is widely used for the prediction of the response to endocrine therapy and as a biologic parameter closely related to disease prognosis. The IHC method is considered to be a specific, sensitive, and economical method for determining ER and PgR status. The authors developed the first rabbit anti-PgR mAb (clone SP2) used in IHC on formalin-fixed, paraffin-embedded tissue sections from breast carcinomas. This new antibody, compared with currently available anti-PgR antibodies, has important advantages, including its reactivity even without heat-based antigen retrieval of fixed-embedded tissue sections in IHC and the predominance of nuclear immunostaining with only very low cytoplasmic signal. A comparative study of IHC on 107 histologic specimens from breast cancer cases showed that SP2 yields the same results as the wellknown mouse mAb to PgR (clone 1A6). The antibody affinity of SP2 is 12 times higher than that of 1A6. Thus, SP2 may prove of great value in the assessment of PgR status in human breast cancer.